The discovery came to light at the beginning of this month, with one member of the gang confessing he had stolen the ‘laissez-passer’ cards from the EU institution.

Insiders at the Ecosoc say the man had been employed by the removal company that undertook the move from its old home on Brussels’ rue Ravenstein to its new headquarters on rue Belliard, which were opened last night (15 September).

They say they were among numerous items that went missing during the move. Computer equipment and a variety of valuable items belonging to staff were also stolen.

One source said the Committee felt obliged under the tendering rules applying to EU institutions to hire the removal company which offered the lowest rates. “There were a lot of breakages and quite a lot of theft,” the source explained. “It seems they broke the bond of trust that you should be able to have with a removal company.”

An internal probe at the Ecosoc is believed to have revealed that the diplomatic passes had been in a secure place prior to the move and were probably stolen in transit. It is understood the theft occurred in June but the Ecosoc only became aware of it when it received a call from an investigating magistrate in Liège on 2 September. The Committee’s head of security Pascal Erauw identified the documents when he travelled to the eastern Belgian city a week later.

Distinguished by a hard blue cover, the laissez-passer cards are normally available to MEPs and senior officials in the EU institutions. But members of the Ecosoc – which was set up by the 1957 Treaty of Rome to give employers and trade unions a say in drafting European Community laws – can apply for them, as can local authority representatives on the Committee of the Regions.

The passes are designed to let holders move swiftly through passport checkpoints at airports, although the European Parliament has been involved in a dispute with the Russian authorities who refused to recognize them when a delegation of MEPs sought to visit Chechnya.

They have also been used by MEPs taking part in demonstrations to evade arrest, particularly in cases where anti-nuclear protesters have blocked transports carrying radioactive substances.